1. Field of the Invention
The present invention relates to a multimeter, and more particularly to a multimeter with a filtered measurement mode.
2. Description of Prior Art
Digital multimeters measure a wide range of electrical parameters, such as AC and DC currents and voltages, and resistance.
However, there are a number of measurement situations confronting the technician in which it is extremely difficult to make accurate measurements. For example,
1. adjustable speed motor drives;
2. uninterruptible power sources; and
3. switching power supplies use pulse-width modulation using high frequency and high voltage pulses to generate pulse-width modulated pulses, or lower frequency sinusoidal signals.
However, the measured root-mean-square (RMS) voltage value is substantially larger than the actual RMS voltage value to cause significant distortion because the measured RMS voltage value contains the high-frequency component, which is caused from factors such as voltage spikes, noise, and switching transients. Hence, a low-pass filter, installed in the multimeter, is used to filter out unnecessary high-frequency component of the voltage signal to acquire correct RMS voltage value.
The U.S. Pat. Nos. 7,342,393 and 7,034,517 both disclosed a multimeter with a filtered measurement mode, and the multimeter has the above-motioned low-pass filtering function to filter out unnecessary high-frequency component of the voltage signal to acquire correct RMS voltage value. However, the disclosed multimeter has a single button on the panel to select an operation of low-pass filtering mode. Also, users can recognize that the low-pass filtering mode is operated while a symbol of using a low-pass filtering mode, such as “LO”, is shown on a view screen of the multimeter. The low-pass filtering mode is canceled and the symbol “LO” is not shown when the single button is pressed again. However, such operation is not simple and convenient.
In practically, users have to carefully check whether the symbol “LO” is shown or not before operating the low-pass filtering mode in order to avoid misemploying the multimeter to acquire unwanted measured results.